Atty. Gen. Janet Reno, whose first deputy resigned over management differences, is getting a replacement with whom she has worked comfortably: Jamie S. Gorelick, now the Pentagon's top lawyer. Reno announced that President Clinton will nominate Gorelick, 43, who helped craft the Administration's policy on gay men and lesbians in the military. She won praise from members of the congressional armed services committees for her straightforward testimony on the issue.